For example, in connecting two air-conditioning ducts, conventionally, an end part of a cylindrical bayonet joint (nipple) is inserted into an end part of the one duct, and the other end part of the bayonet joint is inserted into an end part of the other duct. Then, both insertion parts are subjected to screw driving or the like using an electric power tool. However, in this method, a bayonet joint is indispensable separately in addition to ducts, thereby increasing the cost of component parts. Further, screw driving or the like needs to be given to the two insertion parts, thereby taking time and cost for the work.
Therefore, a spiral pipe is known which has an end part smaller in outer diameter than the pipe itself, thereby dispensing with a bayonet joint (e.g., refer to Patent Document 1). This spiral pipe is formed by shrinking the outer diameter of an end part thereof, making a plurality of grooved concave portions in the shrunk part (the diameter-shrunk end part) and inserting the shrunk part into a spiral pipe to be connected thereto. Consequently, a bayonet joint becomes dispensable.